1. Field of the Invention
The present invention relates to a control apparatus for a voltage transforming apparatus having a transformer and configured to rectify an AC voltage being transformed by the transformer by use of synchronous-rectifying switching elements which are provided in the secondary side of the transformer and are turned on and off in synchronization with power switching elements provided in the primary side of the transformer, and to smooth the rectified DC voltage.
2. Description of Related Art
It is known to provide a voltage transforming apparatus having a transformer with synchronous-rectifying switching elements instead of a diode serving as a rectifying means in the secondary side of the transformer. Since the on-resistance of the synchronous-rectifying switching elements is smaller than that of the diode, replacing the diode with the synchronous-rectifying elements makes it possible to reduce power loss of the voltage transforming apparatus.
However, it should be noted that the voltage transforming apparatus provided with the synchronous-rectifying switching elements cannot always perform their synchronous rectifying control operations, because when the output current of the voltage transforming apparatus is small, there is a fear that the output current flows back through the synchronous-rectifying elements, which lowers the performance and reliability of the synchronous-rectifying elements.
Accordingly, it has been proposed to monitor the output current of the voltage transforming apparatus on the basis of a voltage drop across a current-detecting resistor provided in the secondary side of the transformer, and to inhibit the synchronous-rectifying control operation if the voltage drop indicates that the back flow of the output current can occur.
Incidentally, in recent years, some hybrid vehicles are provided with such a voltage transforming apparatus for the purpose of stepping down a DC output voltage of a high-voltage source, and charging a low-voltage battery by this stepped-down DC voltage. In this case, since a large output current flows in the secondary side of the transformer, the current-detecting resistor produces a large amount of heat. This can cause the voltage transforming apparatus to become large in size. In addition, in this case, the current-detecting resistor is required to have a highly accurate resistance value.
It may occur that a hall element is used instead of the current-detecting resistor. However, using the hall element increases the production cost of the voltage transforming apparatus, because the hall element is expensive.
For such reasons, it has been attempted to estimate the output current flowing in the secondary side of the transformer on the basis of a current flowing in the primary side of the transformer. However, since the current flowing in the primary side of the transformer is an intermittent current, and in addition, it varies depending on the way in which a duty control of the power switching elements provided in the primary side is performed, the output voltage of the high voltage source, and the output voltage of the voltage transforming apparatus, even when the output current of the voltage transforming apparatus is kept constant. Accordingly, it has been very difficult to make the correct judgment as to whether the synchronous-rectifying control operation should be performed or stopped on the basis of the detected value of the current flowing in the primary side of the transformer.